The invention is concerned with increasing the portion of heavy petroleum crudes which can be utilized as catalytic cracking feedstock to produce premium petroleum products, particularly motor gasoline of high octane number. The heavy ends of many crudes are high in Conradson Carbon and metals which are undesirable in catalytic cracking feedstocks. The present invention provides an economically attractive method for selectively removing and utilizing these undesirable components from the residues of atmospheric and vacuum distillations commonly called atmospheric and vacuum residua or "resids". The undesirable CC (for Conradson Carbon) and metal bearing compounds present in the crude tend to be concentrated in the resids because most of them are of high boiling point. The invention provides a method for processing whole crudes high in Conradson Carbon and metals to provide feedstock for catalytic cracking.
When catalytic cracking was first introduced to the petroleum industry in the 1930's, the process constituted a major advance in its advantages over the previous technique for increasing the yield of motor gasoline from petroleum to meet a fast-growing demand for that premium product. The catalytic process produces abundant yields of high octane naphtha from petroleum fractions boiling above the gasoline range, upwards of about 400.degree. F. Catalytic cracking has been greatly improved by intensive research and development efforts and plant capacity has expanded rapidly to a present-day status in which the catalytic cracker is the dominant unit, the "workhorse" of a petroleum refinery.
As installed capacity of catalytic cracking has increased, there has been increasing pressure to charge to those units greater proportions of the crude entering the refinery. Two very effective restraints oppose that pressure, namely Conradson Carbon and metals content of the feed. As these values rise, capacity and efficiency of the catalytic cracker are adversely affected.
The effect of higher Conradson Carbon is to increase the portion of the charge converted to "coke" deposited on the catalyst. As coke builds up on the catalyst, the active surface of the catalyst is masked and rendered inactive for the desired conversion. It has been conventional to burn off the inactivating coke with air to "regenerate" the active surfaces, after which the catalyst is returned in cyclic fashion to the reaction stage for contact with and conversion of additional charge. The heat generated in the burning regeneration stage is recovered and used, at least in part, to supply heat of vaporization of the charge and endothermic heat of the cracking reaction. The regeneration stage operates under a maximum temperature limitation to avoid heat damage of the catalyst. Since the rate of coke burning is a function of temperature, it follows that any regeneration stage has a limit of coke which can be burned in unit time. As CC of the charge stock is increased, coke burning capacity becomes a bottleneck which forces reduction in the rate of charging feed to the unit. This is in addition to the disadvantage that part of the charge has been diverted to an undesirable reaction product.
Metal bearing fractions contain, inter alia, nickel and vanadium which are potent catalysts for production of coke and hydrogen. These metals, when present in the charge, are deposited on the catalyst as the molecules in which they occur are cracked and tend to build up to levels which become very troublesome. The adverse effects of increased coke are, as reviewed above. The lighter ends of the cracked product, butane and lighter, are processed through fractionation equipment to separate components of value greater than fuel to furnaces, primarily propane, butane and the olefins of like carbon number. Hydrogen, being incondensible in the "gas plant", occupies space as a gas in the compression and fractionating train and can easily overload the system when excessive amounts are produced by high metal content catalyst, causing reduction in charge rate to maintain the FCC unit and auxiliaries operative.
These problems have long been recognized in the art and many expedients have been proposed. Thermal conversions of resids produce large quantities of solid fuel (coke) and the pertinent processes are characterized as coking, of which two varieties are presently practiced commercially. In delayed coking, the feed is heated in a furnace and passed to large drums maintained at 780.degree. to 840.degree. F. During the long residence time at this temperature, the charge is converted to coke and distillate products taken off the top of the drum for recovery of "coker gasoline", "coker gas oil" and gas. The other coking process now in use employs a fluidized bed of coke in the form of small granules at about 900.degree. to 1050.degree. F. The resid charge undergoes conversion on the surface of the coke particles during a residence time on the order of two minutes, depositing additional coke on the surfaces of particles in the fluidized bed. Coke particles are transferred to a bed fluidized by air to burn some of the coke at temperatures upwards of 1100.degree. F., thus heating the residual coke which is then returned to the coking vessel for conversion of additional charge.
These coking processes are known to induce extensive cracking of components which would be valuable for FCC charge, resulting in gasoline of lower octane number (from thermal cracking) than would be obtained by catalytic cracking of the same components. The gas oils produced are olefinic, containing significant amounts of diolefins which are prone to degradation to coke in furnace tubes and on cracking catalysts. It is often desirable to treat the gas oils by expensive hydrogenation techniques before charging to catalytic cracking. Coking does reduce metals and Conradson Carbon but still leaves an inferior gas oil for charge to catalytic cracking.
Catalytic charge stock may also be prepared from resids by "deasphalting" in which an asphalt precipitant such as liquid propane is mixed with the oil. Metals and Conradson Carbon are drastically reduced but at low yield of deasphalted oil.
Solvent extractions and various other techniques have been proposed for preparation of FCC charge stock from resids. Solvent extraction, in common with propane deasphalting, functions by selection on chemical type, rejecting from the charge stock the aromatic compounds which can crack to yield high octane components of cracked naphtha. Low temperature, liquid phase sorption on catalytically inert silica gel is proposed by Shuman and Brace, OIL AND GAS JOURNAL, Apr. 16, 1953, page 113.
An improved process for decarbonizing and demetallizing crudes and residual fractions is described in my prior applications Ser. No. 875,326, filed Feb. 6, 1978 (now abandoned) and Ser. No. 038,928, filed May 14, 1979, which issued as U.S. Pat. No. 4,243,514. Those applications describe processes of contacting a resid or a crude oil having an appreciable Conradson Carbon (CC) content and usually a high metals content with an inert solid of low surface area at temperatures above about 900.degree. F. for very short residence times of two seconds or less, preferably less than 0.5 second, separating oil from the solid and quenching the oil below cracking temperature as rapidly as possible. The necessary short residence time is conveniently achieved by supply of the solid in a size of about 20 to 150 microns particle diameter mixed with the hydrocarbon charge in a riser. The oil is introduced at a temperature below thermal cracking temperature in admixture with steam and/or water to reduce partial pressure of volatile components of the charge. The catalytically inert solid is supplied to a rising column of charge at a temperature and in an amount such that the mixture is at a temperature upwards of 900.degree. F. to 1050.degree. F. and higher, e.g. 1250.degree. F., sufficient to vaporize most of the charge.
At the top of the riser the solid is rapidly separated from oil vapors and the latter are quenched to temperatures at which thermal cracking is essentially arrested. During the course of this very short contact, the heavy components of high CC value containing the majority of the metal content are laid down on the solid particles. This deposition may be a coalescing of liquid droplets, adsorption, condensation or some combination of these mechanisms. In any event, there appears to be little or no conversion of a chemical nature. Particularly, thermal cracking is minimal. The quantity removed from the charge under preferred conditions is very nearly that indicated by CC of the feedstock charged. Further, the hydrogen content of the deposit on the solids is believed to be about 6%, below the 7 to 8% normal in FCC coke.
The solids, now bearing deposits of the Conradson Carbon and metals components of the hydrocarbon feedstock, are contacted with a source of oxygen, (air, for example) by any of the techniques suited to regeneration of FCC catalyst, preferably under conditions of full CO combustion to less than 1000 p.p.m. CO in the flue gas. Combustion of the deposited material from the inert solids generates the heat required in the contacting step when the combusted inert solid is recycled to the riser for subsequent contact with the new charge of hydrocarbon feedstock in the contactor. During repeated cycling between the contactor and burner, portions of inert solid are removed from the system and replaced with fresh inert solids in order to maintain a suitable level of metals on the solid while it is in the contactor. Replacement of all or part of inert fluidizable solid for subsequent contact with incoming feedstock charge to the contactor is provided in accordance with my process by utilizing heat in the burner to form the fluidizable particles in situ. This is preferably accomplished by spraying a slurry of a precursor of the inert solid directly into the hot gases in the upper dilute hot gaseous phase of a burner operated with a lower dense phase in a manner such that sprayed material is dried by the hot gases in the burner to form fine beads (microspheres) of inert solid of low surface area.